1. Field of the Invention
The invention relates to an exposure method for exposure of a substrate which is arranged in the area of an image plane of a projection objective by at least one image of a pattern of a mask which is arranged in the area of an object plane of the projection objective, and to a projection exposure system for carrying out this method.
2. Description of the Related Art
Microlithographic exposure methods and projection exposure systems are used for production of semiconductor components and other finely structured parts. They are used to project patterns of photomasks or graduated reticles, which are referred to in a general form in the following text as masks or reticles, onto a substrate which is coated with a radiation-sensitive layer, for example onto a semiconductor wafer which is coated with photoresist, with a very high resolution and on a reduced scale.
A projection exposure system for microlithography comprises an illumination system for illumination of the mask with illumination radiation, as well as a projection objective which follows the mask and which is used to image the pattern of the mask in the image plane of the projection objective. In this case, the radiation which has been changed by the mask passes through the projection objective, which produces output radiation that is directed at the substrate and whose characteristics determine the quality of the image production. In this case, the output polarization state, that is to say the polarization state of the output radiation which emerges from the projection objective and is directed at the substrate, plays an increasingly important role for decreasing wavelengths and increasing numerical apertures.
When using conventional lithography objectives, with image-side numerical apertures NA which are not excessively high, and which objectives have a purely refractive (dioptric) form and are normally operated using unpolarized light at wavelengths of 248 nm or above, the output polarization state is in most cases not critical. For systems which are operated with polarized light, for example catadioptric projection objectives with a polarization-selective, physical beam splitter (beam splitter cube, BSC), the output polarization state is, in contrast, a critical parameter.
Birefringence effects of synthetic quartz glass are significant even at operating wavelengths of about 193 nm. When using fluoride crystal materials, such as calcium fluoride, which are used, for example, in order to avoid compacting and/or for correction of color errors, it should be borne in mind that these materials are polarization-optically effective. Owing to stress-induced and/or intrinsic birefringence, they can cause polarization-changing effects on the light passing through them.
At the moment, only calcium fluoride is available in the required quality and quantity as a lens material for operating wavelengths of about 157 nm or below. At these short operating wavelengths, the influence of intrinsic birefringence is several times stronger than at a wavelength of 193 nm. Stress birefringence is likewise frequently observed to a disturbing extent.
It should also be borne in mind that deflection mirrors are used for many optical systems used in projection exposure systems, can be operated with an oblique radiation incidence and can accordingly produce a polarizing effect. For example, one or more deflection mirrors can be provided in the exposure beam path, that is to say between the light source and the outlet of the exposure system, in order to reduce the physical length of the exposure unit. Owing to the different reflection levels for s-polarized and p-polarized field components of the radiation coming from the light source, it is possible, for example, for partially polarized illumination radiation to be produced from initially unpolarized radiation. If linear-polarized laser light is used, then the direction of the linear polarization can be changed, or an elliptical polarization state can be produced by using an appropriate phase effect. In the case of catadioptric systems, obliquely illuminated deflection mirrors are likewise frequently provided in the area of the projection objectives, which may have a polarization-changing effect, and accordingly influence the output polarization state.
For high numerical apertures, for example with values of NA=0.85 or more, the vector character of the image-producing electric field also makes itself increasingly noticeable. For example, the s-polarized components of the electric field, that is to say that component which oscillates at right angles to the incidence plane spanned by the incidence direction and the normal to the surface of the substrate interferes better and produces better contrast than the p-polarized component which oscillates at right angles to it. In contrast, p-polarized light is generally coupled better into the photoresist. Proposals have therefore already been made to operate with specifically polarized output radiation, for example with tangential polarization or radial polarization, depending on the use of high apertures. Sometimes, even circular-polarized or unpolarized output radiation is also desirable.
Unfavorable polarization states can lead to a variation in the width of imaged structures over their direction. Such interference with a desired direction independence of the image is frequently referred to as HV differences or critical dimension variation (CD variation). Variations of the imaged structure widths across the field are also observed. Furthermore, undesirable nonlinear relationships may occur between the size of the structure to be imaged and the size of the imaged structure. In addition, unfavorable polarization states can induce telecentricity errors, which can lead to undesirable distortion between different adjustment planes. Not least, in systems which operate with polarization, radiation of the parasitic polarization which, for example, can occur due to leakage transmission on polarizing elements, may have a contrast-reducing effect.
European Patent Application EP 0 937 999 A1 discloses a microlithographic projection objective, which contains one or more optical elements which cause disturbance of the polarization distribution across the cross section of a light beam. This disturbance of the polarization distribution is at least partially compensated for by means of a polarization compensator, which comprises at least one birefringent optical element with a thickness which varies irregularly over its cross section. The polarization compensator has a fixed predetermined spatially varying effect function, is manufactured individually in the form of “polarization goggles” on the basis of polarization-optical measurement data which is recorded on the completely assembled and adjusted system, and is permanently mounted in the system by the manufacturer.
EP 964 282 A2 deals with the problem of a preferred polarization direction being introduced when light passes through catadioptric projection systems with deflection mirrors, resulting in the deflection mirrors, which have two or more coatings, having different deflection levels for s-polarized and p-polarized light. In consequence, light which is still unpolarized in the reticle plane is partially polarized in the image plane, which is said to lead to direction dependency of the imaging characteristics. This effect is counteracted by producing partially polarized light in the illumination system with a predetermined residual polarization degree in order to create a polarization advance, which is compensated for by the projection optics such that unpolarized light emerges at its output.
EP 0 602 923 B1 (corresponding to U.S. Pat. No. 5,715,084) discloses a catadioptric projection objective which is operated with linear-polarized light and has a polarization beam splitter, in which a device for changing the polarization state of the light passing through is provided between the beam splitter cube and the image plane, in order to convert the incident, linear-polarized light to circular-polarized light (as an equivalent to unpolarized light). This is intended to ensure imaging contrast that is independent of the structure direction. A corresponding proposal is also made in EP 0 608 572 (corresponding to U.S. Pat. No. 5,537,260).
The U.S. Pat. No. 5,673,103 discloses a projection illumination method for reticle structures with at least two different structure directions, which are intended to be imaged with a preferred polarization direction using polarized light. A rotatable polarization control device is used to align the preferred polarization direction of the illumination radiation for each structure direction, by rotation optimally with respect to the structure direction.
The U.S. Pat. No. 5,922,513 describes a projection exposure method which operates with elliptically polarized light. On the basis of theoretical considerations, this document proposes that the ellipticity degree and the ellipticity angle be set as a function of the reticle structures so as to produce optimum contrast.